I think its a condition of purchasing a ticket normally, that they can replace a fighter if required. Probably hidden in the T&Cs.
Big promoters should offer refunds though tbh. When Whyte pulled out against Joshua and Helenius replaced him, they offered refunds. The funny thing is that when tickets went on sale for Wardley vs Miller, the fight wasn't actually signed yet. Miller said they were still working on his contract. The cynic in my thinks they couldn't agree on money which is why Miller pulled out.